Combobox: Allow PopperJS to position menu correctly

[interactivellama]

Fixes #1451 This enables dialog flip and preventing overflow by adding height and width to Combobox menu

Additional description

.slds-dropdown sets the menu to absolute positioning, since it has a relative parent. Absolute positioning removes clientHeight and clientWidth which Popper.js needs to absolute position the menu's wrapping div. Absolute positioning an already absolute positioned element doesn't work. Setting the menu's position to relative allows PopperJS to work it's magic.

https://design-system-react-co-pr-1454.herokuapp.com/?selectedKind=SLDSModal&selectedStory=Small%20with%20footer%2C%20not%20dismissible&full=0&addons=1&stories=1&panelRight=0&addonPanel=storybook%2Factions%2Factions-panel

@qyiming Please pull down and review in your sidebar.

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REVIEWER checklist (do not remove)

• TravisCI tests pass. This includes linting, Mocha, Jest, Storyshots, and components/component-docs.json tests.
• Tests have been added for new props to prevent regressions in the future. See readme.
• Review the appropriate Storybook stories. Open http://localhost:9001/.
• The Accessibility panel of each Storybook story has 0 violations (aXe). Open http://localhost:9001/.
• Review tests are passing in the browser. Open http://localhost:8001/.
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[interactivellama]

@vintik This shouldn't change any of your code (I think), but it affects dialog positioning for Combobox.

[qyiming]

@interactivellama I tested your PR in my "sidebar", the dialog is now able to flip correctly. We do not need #1452 any more.